There's much code duplication of CSS for styling Special:Preferences in skins. This should ideally be a MediaWiki core module, like `mediawiki.special.preferences`. However, if T117781 is accomplished, this task might have to be declined.
* #monobook: https://github.com/wikimedia/mediawiki-skins-MonoBook/blob/master/skinStyles/mediawiki.special.preferences.styles.less - every other skin on the list duplicates MonoBook's CSS for preferences
* #Bouquet: https://github.com/wikimedia/mediawiki-skins-Bouquet/blob/master/resources/style.css#L840
* #deepsea: https://github.com/wikimedia/mediawiki-skins-DeepSea/blob/master/deepsea/screen.css#L605 (copy of Vector, which copies MonoBook w/ some differences)
* #Dusk: https://github.com/wikimedia/mediawiki-skins-Dusk/blob/master/resources/css/main.css#L338
* #dusktodawn: https://github.com/wikimedia/mediawiki-skins-DuskToDawn/blob/master/resources/style.css#L689
* #Gamepress: https://github.com/wikimedia/mediawiki-skins-Gamepress/blob/master/resources/css/style.css#L927
* #metrolook: https://github.com/wikimedia/mediawiki-skins-Metrolook/blob/master/skinStyles/mediawiki.special.preferences.styles.less (copy of Vector, which copies MonoBook w/ some differences)
* #refreshed: https://github.com/wikimedia/mediawiki-skins-Refreshed/blob/master/refreshed/preferences.css
* #Truglass: https://github.com/wikimedia/mediawiki-skins-Truglass/blob/master/truglass/handheld.css#L788
* #vector: https://github.com/wikimedia/mediawiki-skins-Vector/blob/master/skinStyles/mediawiki.special.preferences.styles.less (copies Monobook but with some differences)